Heat-sealable plastic resins, such as propylene-ethylene copolymers, are often used to manufacture packages that can be rapidly sealed upon rapid application of heat. In sealing, typically two sides of a closure are heated such that they interdiffuse to form a seal. The seal remains integral from the time of closure until the time of use, thus preserving product quality. Rapid sealing reduces the cost for producing the package, such as, for example, a bag for food.
In addition to exhibiting desirable levels of sealing strength and sealing speed and temperature, it is also desirable that the heat-seal resins exhibit a low level of extractables, i.e., a low level of substances in the polymer matrix that solubilize and permeate into the contents of the package upon contact therewith. A low level of extractables is particularly important when food products are being contained but is difficult to attain, as some substances in food products may act as a solvent for the extractables.
Selecting a heat-seal resin or blend of resins involves balancing of polymer physical properties. A high strength of seal typically correlates with high crystallinity and high molecular weight for the polymer. A low level of extractables in the resin correlates to high crystallinity (inverse relationship). In contrast, a low temperature of sealing, which facilitates rapid seal initiation at low temperatures and low energy, correlates to low crystallinity and low molecular weight. The temperature at which sealing is initiated is referred to as the “seal initiation temperature” or SIT. Balancing of properties typically requires the use of blends of two or more resins of different physical properties.
One method employed in the art to provide a proper balance of polymer physical properties in heat-seal resins is to use Ziegler-Natta copolymers and terpolymers of broad orthogonal composition distribution (BOCD). BOCD copolymers and terpolymers are disclosed, for example, in U.S. Pat. Nos. 5,382,630; 5,382,631; 5,530,065; 6,969,741; 7,022,770; 7,307,126; 7,396,878; and 7,396,881. A disadvantage of heat-seal resins made using Ziegler-Natta catalysts is deficiency in performance in one or more physical properties. Heat-seal resins made using Ziegler-Natta catalysts are known to produce broad, uncontrolled distributions with respect to both molecular weight and comonomer content. A disadvantage of terpolymers using Ziegler-Natta catalysts is high production cost.
It would be desirable to have a resin blend that exhibited high seal strength, a low level of extractables in the resin, a low seal initiation temperature (SIT). It would also be desirable to have an alphaolefin copolymer blend that exhibited such properties.